Imagine coming upon a lot of delicious-looking mushrooms, a couple of which are in the hands and mouth of a dead person. Being faster to realize the person is dead, and to flee, than the time it takes to take and eat the mushrooms, is why you'll live to reproduce. Same for the dead animals around that fresh-looking water hole.

I seem to recall that animals can sense whether other animals are dead (or rotten/contaminated)?

They smell putrefaction.

It starts immediately when life has ended. It doesn't wait for days, or hours, out of decency or so...

I don't think there's any great mystery to this. We want to breed with the mate most likely to help us produce viable offspring. Therefore we're sensitive to indications of health and good genes. Symmetry, smoothness of muscle movements, quality of skin & hair, indications of good blood flow, even things like regular breathing are all indications. When we see a simulation that appears not quite human it's noticable for all those details. That detection of unhealthy can easily detect death of course. There may also have been an instinct to avoid the sick, but social pressures override that sometimes.

The internet theory that we wanted to detect imposter humans is silly. Early hominids interbred a lot. It's not "human" that we're sensitive too. Just health.

I love this question! I’ve seen it asked a few different ways. “What evolutionary benefit did the recoil reaction to the ‘uncanny valley’ provide?”. Thinking about the answer is kinda scary.

I’d think one benefit of being able to quickly discern between the living and the dead would be increased survival chances during death events. If you can see that someone is dead, less likely to face a dangerous predator, natural disaster, or contagion, or at least stop investing precious time in those situations if they found themselves in them.

I'm not an expert on this at all, but my understanding is that "evolutionary advantage" is a misconception. Mutations don't have a goal, and they don't always provide an advantage. Hopefully someone smarter than me can explain better.

Edit: spelling